Westbound, seems to me that the best way on OW is SIN-HKG-JNB on CX, then JNB-MRU on BA. You could also, of course, go SIN-SYD-JNB (QF), but much more time and money. And, if you don't care about the OW mileage, then SIN-JNB on SQ, JNB-MRU on BA, for only one cnx instead of two.